Dopamine modulation of pathological human chromaffin tissue.

To evaluate the presence of dopamine-2(DA2) binding sites in pheochromocytoma tissue, we performed binding studies and light microscopy autoradiography on sections of 7 different tumors. 3-H-Spiroperidol, a DA2 ligand, bound specifically to tumor sections with a (mean +/- SD) Kd value of 1.93 +/- 0.62 nmol/L. Binding site density (Bmax) was 29.16 +/- 2.33 fmol/mg tissue. Light microscopy autoradiography showed a nonhomogenous localization of silver grains within chromaffin cells. The specific binding was about 50% of total. To investigate whether DA2 binding sites found on pheochromocytoma cells might modulate catecholamine (CA) release, we studied the effects of oral bromocriptine (2.5 mg) on circulating CA of 5 patients with pheochromocytoma. In these patients bromocriptine caused a decrease in blood pressure (P less than .05) but no significant change in plasma CA. Our study shows the presence of DA2 binding sites on tumoral chromaffin tissue. As bromocriptine, a DA2 receptor agonist, was not able to modify tumor CA secretion, the functional role, if any, played by these binding sites on tumor secretion is still to be clarified.